Form stepper: component updates

[Spark450]

Acceptance Criteria:

• When in the "mobile" vertical view the Next and Previous buttons should be full width and stacked vertically
• Responsiveness seems to be "broken" from 1044px to 620px, suggest displaying the vertical stepper orientation from 1044px
• In the "controlled navigation" example the user should not be able to navigate with to any step that is "not started" using the "next" button (as stated in the docs).
• @ArakTaiRoth we should discuss this criteria: "A step that is “Not started” will not be clickable." I think that we might need this to be clickable for accessibility reasons.